fix: remove tsutils and tsutils-etc

[JasonWeinzierl]

• Remove unmaintained tsutils and tsutils-etc.
  • tsutils has been replaced by the community with ts-api-utils. See Help maintaining this project? ajafff/tsutils#145. Replacing it was fairly simple.
  • tsutils-etc relies on tsutils, and I haven't seen a community-chosen replacement, so we're re-implementing the utilities into this repo, similar to how we re-implemented eslint-etc utilities. So utilities now exist in ts-api-utils, but couldBeType and couldBeFunction had to be ported over.
• Pull in a dev dependency on @typescript/vfs to unit test couldBeType based on the tsutils-etc unit tests for that function.
• Bump the minimum versions of TypeScript to >=4.2.0 to align with ts-api-utils and tslib to ^2.1.0 to align with rxjs.

[github-actions]

LCOV of commit 548e5f3 during .github/workflows/ci.yml #15

Summary coverage rate:
  lines......: 96.2% (3249 of 3376 lines)
  functions..: 92.9% (196 of 211 functions)
  branches...: 91.2% (613 of 672 branches)

Files changed coverage rate:
                                         |Lines       |Functions  |Branches    
  Filename                               |Rate     Num|Rate    Num|Rate     Num
  =============================================================================
  src/etc/could-be-function.ts           | 100%      8| 100%     1| 100%      4
  src/etc/could-be-type.ts               |97.7%     88| 100%     4|97.7%     44
  src/etc/get-type-services.ts           | 100%     76|73.3%    15|92.0%     25
  src/etc/index.ts                       | 100%      7|    -     0|    -      0
  src/rules/no-unsafe-subject-next.ts    | 100%     53| 100%     1| 100%     14
  src/rules/throw-error.ts               | 100%     51| 100%     3| 100%      7